A motor vehicle contains numerous inductive loads, such as for example electric motors. These electric motors may also be used in electronic actuator control systems. This may be for example an electronic throttle control (ETC) device, or an exhaust gas recirculation (EGR device) valve, or any other valve used in engine control. More generally, it may be any other apparatus actuated by an electric motor, such as a power window for example.
The power supply of such inductive loads generally uses a switching structure, such as a bridge of switches positioned in an H. An H-bridge comprises four power switches, namely two “high-side” switches on the side of a positive power supply, for example a battery, and two “low-side” switches on the side of a negative power supply or of an electrical ground of the motor vehicle. Each switch generally includes a power MOS (“metal-oxide semiconductor”) transistor.
A sequence of analog control signals for the four switches is produced from a setpoint control signal. The setpoint control signal and the analog control signals are generally pulse-width-modulated signals, or PWM signals. The duty cycle of the PWM signals drives the amount of current injected into the inductive load and therefore, on average, the intensity of the current in this inductive load.
Depending on the control strategies, the H-bridge is actuated in given and authorized configurations. By contrast, other configurations are prohibited, such as for example a configuration in which a high-side switch and a low-side switch would be closed together, creating a short circuit between the battery and ground.
However, when controlling the switches of the H-bridge, it is possible for short circuits to occur either at an output of the H-bridge or in the inductive load. Discriminating a short circuit to ground or to the battery at an output of the H-bridge with respect to a short circuit of the inductive load (short-circuited electric motor winding) is not a trivial matter. Specifically, simply detecting a short circuit by way of a current measurement at each transistor sometimes does not make it possible to ascertain whether the short circuit originates from contact of an output with ground or with the battery or from the inductive load.
Numerous techniques are known to those skilled in the art for detecting a short circuit in the inductive load. Mention may be made for example of one solution that consists in applying a current in a direction opposite to the direction of the nominal operating current of the transistors of the H-bridge, that is to say the direction of the current in which the short circuit has been detected. To this end, the two transistors complementary to the two transistors that are actuated in the nominal operating direction are activated for a relatively short time. Of course, the two transistors used for nominal operation of the H-bridge are inactive in this case. If a short circuit is detected upon activation of the two complementary transistors, then this means that the short circuit that is detected originates from the inductive load. However, applying a reverse current to the inductive load causes the latter to drift, which may be detrimental in some cases.
Furthermore, such a method does not exhibit very good performance in the case of intermittent short circuits, which may lead to an erroneous conclusion as to the location of said short circuits.